Laser microsurgery and coronary angioplasty require precise removal of tissue without thermally damaging the surrounding tissues. Such medical/surgical lasers require using a wavelength in the mid-infrared wavelength region, i.e. between about 1.0 and 3.0 microns, which is the wavelength range most strongly absorbed by animal tissue. The 2.94 micron wavelength of the Er:Yag laser is the most well-absorbed by animal tissue and thus the Er:Yag laser is preferred for many surgical procedures.
The tissue absorption coefficient (.alpha.=1000 cm.sup.-1) is the highest with the Er:Yag laser. The .alpha. of other existing medical lasers ranges from 4 cm.sup.-1 for a He-Ne laser to 600 cm.sup.-1 for a CO.sub.2 laser. Thus, the Er:Yag is widely m- 1 for a C02 viewed as the best surgical laser (see "Laser Evolution", Nov. 1991). To efficiently use the power delivered by the Er:Yag laser, a ruggedized infrared (IR) optical fiber which transmits at 2.94 microns must be used. Coupled to the output of the laser beam, the IR fiber can deliver the power to ablate tissues.
Infrared transmitting zirconium fluoride based glasses such as ZrF.sub.4 --BaF.sub.2 --LaF.sub.3 --AlF.sub.3 --NaF--PbF.sub.2 (see Esterowitz et al, "Angioplasty With a Laser and Fiber Optics at 2-94 .mu.m") have emerged as the materials of choice for the IR transmitting fiber because their optical transmission between 2.5 microns to 5.0 microns exceeds 90 percent, as shown in FIG. 1. Similarly, fluoride glass fibers exhibit optical losses as low as 0.06 dB/m at 2.94 microns. This is equivalent to a transmission of over 90 per cent in a one foot long fiber. A flexible fiber hand-piece with a minimum length of 6 inches and a desirable length of one foot or greater can be made adaptable to many laser delivery systems.
However, the zirconium fluoride based glass fibers have a drawback in that they are capable of carrying only very low power. Investigations recently carried out at several medical laser companies have shown the following: a 300 micron core fluoride glass fiber, when coupled to an Er:Yag laser operating with an output power of 180 mJ at a repetition rate of 10 Hz, could transmit around 135 mJ or 1.35 watts but failed after only two minutes. As the laser output power increased, the fiber damage occurred much faster. Fiber damage always occurred at the fiber input end face or along the fiber length resulting in a localized melt-down of the glass followed by power rupture. As a result, zirconium fluoride based glass fibers have a very limited use in conjunction with the Er:Yag laser, and in effect can only be used for a very short time because fiber damage quickly occurs.
It is known that AlF.sub.3 used as a dopant for ZrF.sub.4 base glass can increase the Tg of the glass considerably. However, the quantity of AlF.sub.3 which can be incorporated as a dopant is very low, as AlF.sub.3 tends to destabilize the glass and make it difficult to form into fibers. Even when formed, such fibers are not stable, as the resultant AlF.sub.3 containing glass fibers tend to easily crystallize under localized heating which would inevitably occur with laser usage, thus inducing fiber damage and failure. Therefore, the addition of AlF.sub.3 to the conventional ZrF.sub.4 glass is not a solution to the problem of localized melt-down and resultant failure of the fiber. AlF.sub.3 based glass of about 30 mol% AlF.sub.3 is also known, but this glass is very unstable and difficult to form into fibers.